Auxiliary thermal element for altering trip unit characteristics



May 7, 196s Filed May 2, 1966 7: can?.

C. E. GRYCTKO AUXILIARY THERMAL ELEMENT FOR ALTEHING TRIP UNIT. CHARACTERISTICS 2 Sheets-Sheet l May 7, 1968 c E. snvc'rxo AUXILIARY THERIAL'ELEKENT lFOR A LTERING TRIP UNIT CHARACTERISTICS Filed May 2, 1966 2 Sheets-Sheet 2 United States Patent O 3,382,334 AUXILIARY THERMAL ELEMENT FOR ALTERING TRIP UNIT CHARACTERISTICS Carl E. Gryctko, Haddon Heights, NJ., assigner to I-T-E Circuit Breaker Company, Philadelphia, Pa., a corporation of Pennsylvania Filed May 2, 1966, Ser. No. 546,989 6 Claims. (Cl. 20o-116) ABSTRACT F THE DESCLSURE A circuit breaker trip unit having a heater heated by the circuit being protected; a primary and a secondary thermal element thermally mounted on the heater and thermally mounted independently from each other; the thermal elements being positioned to deliect toward one another as they are he-ated and detlecting as different functions of the load current in the heater; a means interposed between the thermal elements, whereby the delicotion of one thermal element is modified by the deflection of the other, circuit breaker Contacts being normally in engagement and means for separating the contacts being held from operation by a latch supported by a rotatable tripper bar; movement of the thermal elements affecting the rotative position of the tripper bar to release the latch and trip apart the circuit breaker contacts.

This invention relates to an auxiliary thermal element for yaltering the characteristics of a circuit breaker 'trip unit and more particularly to such an auxiliary thermal element which rdeilectsl as a function of circuit breaker load current to alter the thermal characteristics of the primary thermal element.

Every electrical circuit has a maximum rated current value. lf an overload current greater than this value passes through the circuit, electrical devices may burn out, the heat generated can damage the wiring insulation within the circuit and a lire may develop. In order to protect against the dangers of a current overload, a circuit breaker is series connected within the circuit. The current passes into a line input terminal on the circuit breaker, through cooperating contacts in the circuit breaker mechanism and through a load responsive trip unit of the circuit breaker, continuing out of a load terminal of the circuit breaker to the circuit load.

In the event of a current overload in the circuit being protected, the circuit breaker has an automatic mechanism for separating the normally engaged cooperating contacts. This breaks the circuit land immediately terminates the overload current flow. The mechanism of the circuit breaker must be automatically tripped when the amperage becomes excessive. Trip units built into circuit breakers perform -this function.

For sudden overload currents of great magnitude, trip units are generally provided with an instantaneous electromagnetic tripping means. An electromagnet is connected in series with the circuit being protected. An

armature is mounted such that when the electromagnet is suddenly energized by -a very high overload current, the armature is instantaneously attracted by the energized electromagnet. In its movement toward the electromagnet, the armature strikes and moves the hereinbelow dcscribed tripper bar which causes the circuit breaker to separate the cooperating contacts breaking the dangerously overloaded circuit.

If, however, there is only a moderate overload current passing through the circuit being protected, there is no danger of overheating or lire unless the overload condition continues for a considerable period of time. A moderate overload current does not energize the electromag- 3,382,334 Patented May 7, 1968 net suliciently to attract its Iarmature enough to trip the circuit breaker. To trip a circuit breaker after a moderate current overload passes through the circuit being protected for a period of time, a trip unit has a thermal element, eg. a bimetallic strip, the two facing metal strips of which have different coeliicients of expansion. This thermal element may be thermally mounted on a portion of the wiring of the circuit called the heater. By being so mounted, the thermal element can only be indirectly heated by the heat from the heater passing into the element. As greater or lesser amperages pass through the circuit, the natural resistance of the circuit wiring will cause the heater to heat up to a greater or lesser extent. This varies the temperature of the thermal element thermally mounted thereon. As the temperature of the heater increases, the thermal element deects to a greater extent.

Alternatively, the thermal element may be directly connected in series with the circuit being protected, the thermal element being iixedly mounted at one end thereof and having the circuit leading into one end of the thermal element and leading out of the other end thereof. With this arrangement the circuit wiring need not heat up to trip the circuit breaker. Only the thermal element itself must heat up.

The circuit breaker has a latch which, when moved, trips open the cooperating contacts. The latch engages a latch support plate located on a tripper bar. When either the armature being attracted by the electromaguet in the event of a high overload current or the bimetallic strip being deccted by an increase in the temperature in the heater press against the tripper b-ar, they rotate it, which simultaneously moves the latch support plate away from the latch it supports. When the latch slips away from the plate, it rapidly falls, causing the cooperating contacts in the circuit breaker to separate.

A standard trip unit, therefore, is comprised of an electromagnet connected in series with the circuit being protected and a dcilecting armature which is attracted by the electromagnet and of a heater upon which is thermally mounted a bimetallic strip or of a bimetallic strip which 'is directly connected in series in the circuit being protected, and of a tripper bar which is rotated by the deilecting armature or bimetallic strip and of a latch plate to support the latch of the circuit breaker.

An example of a circuit breaker having the above-described trip unit can be seen in FIGURE 3 of U.S. Patent No. 3,213,232, issued on Oct. 19, 1965, to the instant inventor and assigned to the assignee of the instant invention. The present invention is concerned solely with the thermal trip mechanism of the trip unit.

Since a trip unit responds to the temperature of its heater, it is desirable that no other sources of heat alect the bimetallic strip. Circuit breakers, however, are often mounted outdoors. Temperature variations between night and day and seasonal variations in the air temperature will have an effect on the bimetallic strip in the circuit breaker trip units. To render nugatory the effect on the primary circuit breaker trip unit deflecting bimetallic strip of ambient temperature variations within the trip unit, an auxiliary bimetallic strip is operatively positioned to counter the deflection of the primary bimetallic strip due to changes in the ambient temperature. Such auxiliary bimetallic strips are usually mounted on the trip unit casing or on an insulating block therein. In no case would they be mounted on the heater in the trip unit since they are designed to react to the ambient temperature within the trip unit and not to the level of current passing through the heater.

The auxiliary bimetal of the instant invention is not designed for the purpose of compensating for variations in the ambient temperature. Rather, like the primary bimetal, it is designed to respond to variations in the load current passing through the trip unit and it is designed to alter the deflection characteristics of the primary bimetal.

The primary bimetal of a trip unit is designed to etlect tripping of a circuit breaker at pre-determined time intervals when subjected to a moderate overload, as for example: l seconds at 300%, 14 minutes at 200%, 1 hour at 125%. It has been found, however, that a primary bimetal having the characteristic of not tripping the circuit breaker when the circuit is carrying its maximum rated value current (100%) may not have the desired time characteristic of tripping the circuit breaker after a particular magnitude of overload current has passed through the circuit for its maximum safe period. Were an attempt to 'be made to adjust the calibration of the primary bimetal to obtain the desired tripping speed for load currents in excess of 100% of capacity, such calibration might cause the primary bimetal to trip the circuit breaker at currents at or below 100% of the circuits capacity. Furthermore, an adjustment of the primary bimetal to cause tripping at the appropriate speed for a certain overload current might ruin the calibration of the primary bimetal for tripping of the circuit breaker after a higher or lower overload current has passed through the circuit for an appropriate time.

The present invention is designed to properly calibrate the primary bimetal of the thermal trip unit so that the primary bimetal will not trip the circuit breaker when currents at or below 100% of the current carrying capacity of the circuit pass through it, while at the same time permitting or causing the primary bimetal to trip the circuit breaker after appropriate periods of time have elapsed during which various overload currents have passed through the circuit being protected.

Proper adjustment of the calibration of the primary bimetal of the thermal trip unit is accomplished by employing an auxiliary or a plurality of auxiliary bimetals in any one of the following ways.

First, a primary bimetal having the desirable characteristic of not tripping a circuit breaker in which it is positioned at currents below 100% of the current carrying capacity of the circuit may have the undesirable characteristic of also not tripping the circuit breaker after an appropriate period has elapsed during which current at 300% of the capacity of the circuit has passed through the circuit. An auxiliary bimetal can be mounted in accordance with the teachings of the instant invention to adjust the calibration of the primary bimetal such that it will trip the breaker after the appropriate period ot operation at overload current has elapsed.

Alternatively, a primary bimetal having the desirable characteristic that it properly trips the circuit breaker it is mounted in after an appropriate period has elapsed during which an overload current of, say, 300% of the current carrying capacity of the circuit, has passed through the circuit for an appropriate period, might also have the undesirable characteristic of tripping the circuit breaker at a current level below 100% of the capacity of the circuit being protected.

ln this situation, an auxiliary bimetal would be mounted in accordance with the teachings of the instant invention having such characteristics that it would not obstruct the proper operation of the primary bimetal when an overload current of 300% of the current carrying capacity of the circuit passes through the circuit breaker. At the same time, the auxiliary bimetal employed would have such characteristics that it would obstruct the tripping movement of the primary bimetal at currents below 100% of the current carrying capacity of the circuit being protected. For instance, it may be desirable to protect a circuit which carries 700 amperes continuously and must be interrupted after a period of between 7 and 21 seconds has elapsed at 2100 amperes, or 300% of the rated current. It is then found that it is difficult to provide a trip unit or thermal element with the necessary deflection and torque characteristics to maintain 700 amperes continuously and trip the circuit breaker as required. It is, however, realized that a 600 ampere trip unit incorporates a thermal element that will trip the breaker in the fast time of 7-21 seconds at 2100 amperes. It follows that the 600 ampere bimetal will do the job at overload conditions, but will undesirably cause tripping at 700 amperes. It is here that the present invention comes into play in providing an auxiliary bimetal which retards (after a considerable time delay) the primary bimetal at rated 700 ampere current, but is not adequate or active enough to retard the required protection and tripping at overload conditions.

As will Ibecome apparent after reading below the detailed description of one embodiment of the present invention, a plurality of auxiliary bimetals may also be employed for Calibrating the primary bimetal. Each of the auxiliary bimetal-s would appropriately adjust the calibration of the primary bimetal for diiterent level currents passing through the circuit being protected. The plurality of auxiliary bimetals might be necessary because diterent primary and auxiliary bimetals would have dierent deection characteristics and, hence, different trip affecting characteristics 'at the various current levels which might pass through the circuit being protected.

The present speciication will be concerned with an embodiment where a single auxiliary bimetal is applied to alter the trip characteristics of a primary bimetal which trips the circuit breaker after an overload current has passed through the circuit for an appropriate period, but which primary bimetal also trips the circuit breaker at current levels below the maximum rated current level of the circuit being protected. After a reading ofthe description appearing below, the employment of the present invention in other situations requiring calibration of the primary bimetal of the circuit breaker tri-p unit will become apparent. Y

The description below is concerned with a means to prevent tripping of a circuit breaker through deflection of its primary bimetal after continued operation of the breaker at or below of its current carrying capacity without affecting the trip characteristics of the primary bimetal under overload conditions. To accomplish this, an auxiliary bimetal is either thermally mounted on the trip unit heater or thermally mounted by being directly connected in series with the circuit being protected. The auxiliary bimetal is so positioned that when it is heated, it will bend toward the thermally mounted primary bimetal. When the primary bimetal is heated, it bends toward the auxiliary bimetal. An intervening mechanism permits the bending of the auxiliary bimetal to be translated into a restraining force against the bending of the primary bimetal. In this manner, the auxiliary bimetal prevents the primary bimetal from tripping the circuit breaker at normal current loads.

The auxiliary bimetal must deiiect as a diiterent function of the current passing through the heater than the primary bimetal. It must both operably bend at less than 100% of the current rating of the breaker and it must react `more slowly than the primary bimetal to overload currents. An auxiliary bimetal possessing the former characteristic will be eliective to prevent tripping of the breaker after long-term current ow at or below 100% of current carrying capacity. An auxiliary bimetal with the second characteristic will not prevent proper operation of the trip unit if an overload current occurs.

The auxiliary bimetal may be positioned Vsuch that it would deect in the same direction as the primary bimetal when the current passing through the circuit being protected increases. If the auxiliary bimetal deflects as a different function of the load current than the primary bimetal, and if the two bimetals are mechanically linked, then as the auxiliary bimetal deflects and urges the primary bimetal in the direction of its deiiection, the trip characteristics of the trip unit will be altered. For instance, the auxiliary bimetal could urge the primary bimetal to deect faster than it normally would at lower current ratings without interfering with the deflection of the primary bimetal at higher current ratings. Conversely, the auxiliary bimetal could be inactive at low currents, but help and speed up the deflection of the primary bimetal and, hence, speed up the tripping at higher overload currents.

Accordingly, it is a primary object of the present invention to provide an auxiliary thermal element or bimetal for a circuit breaker trip unit which will alter the deflection characteristics and, thereby, the circuit breaker tripping characteristics of the primary thermal element or bimetal.

It is a second object of the present invention to provide an auxiliary bimetal for a trip unit which will prevent the primary bimetal of the trip unit from tripping the circuit breaker after long-term operation at less than overload currents while not affecting the deflection characteristics of the primary bimetal at overload currents.

It is a third object of the present invention to provide an auxiliary bimetal for a trip unit which will not affect the deflection of the primary bimetal at less than overload currents but which will affect the deflection characteristics of the primary bimetal in response to an increase in the current flowing through the circuit being protected above the maximum rating of the circuit.

It is another object of the present invention to provide an auxiliary bimetal for a trip unit which will aifect the deflection characteristics of the primary bimetal and, hence, which will adjust the calibration of the primary bimetal, both at currents below and above overload passing through the circuit being protected.

It is a further object of the present invention to provide a circuit breaker trip unit employing a primary bimetal with certain deflection characteristics and a plurality of auxiliary bimetals. each with deflection characteristics different from that of the primary bimetal and different from that of one another to alter the calibration of the primary bimetal and thereby affect the tripping characteristics of the trip unit for currents greater than and less than the maximum rated current value of the circuit being protected. It is another object of the present invention to provide an auxiliary bimetal for a trip unit, in accordance with the foregoing objects, which is thermally mounted on the same heater as the primary bimetal.

It is another object of the present invention to provide an auxiliary bimetal for a trip unit, in accordance with the foregoing objects, which with the primary bimetal is directly connected in series with the circuit being protected.

Itis a further object of the present invention to provide an auxiliary bimetal for a trip unit which obtains the characteristics differentiating it from the primary bimetal in part from the manner in which it is mounted.

These and other objects of the present invention will become apparent after reading the following description of the accompanying drawings in which:

FIGURE 1 is a cross-sectional view through one embodiment of a trip unit containing the instant invention.

FIGURE 2 is a cross-section through a circuit breaker containing the trip unit of FIGURE l.

FIGURES 3-5 plot deflection as a function of time for both the primary bimetal and the auxiliary bimetal at three different current ratings passing through the trip unit.

Refer to FIGURE 1. The primary bimetal 11 is comprised of at least two face weldedy strips of metal. The metal nearest the hereinafter described auxiliary bimetal must have a smaller coeicient of expansion than the metal farthest therefrom.

The primary bimetal 11 is thermally mounted, e.g. by rivets 12, on the heater 13. The heater 13 is in series with the circuit being protected. Primary bimetal 11 may be in direct contact with heater 13 or may be separated therefrom, eg. by dielectric insulating block 14. The thermal mounting must be such that sufficient heat reaches the primary bimetal to cause it to deflect in direction A to operate the trip unit when an overload current passes through the heater. The primary bimetal may alternatively be directly connected in series with the circuit being protected. The circuit wiring would lead into the mounted end of the bimetal and would lead out of the unmounted end thereof.

Auxiliary bimetal 15 also consists `of two face welded lengths of metal. The metal on the side of the auxiliary bimetal 15 nearest the primary bimetal 11 must have a small-er coeiiicient of expansion than the metal on the side of the auxiliary bimetal farthest from the primary bimetal. Auxi-liary bimetal 15 is also thermally mounted on `hea-ter 13. It, too, can be directly connected with heater 13 or be separated therefrom, e.g. by an insulating block. Insulating block 16 should be comprised of a dielectric material which is sufficiently heat conductive to permit the auxiliary bimetal to become heated when the heater 13 heats up. The auxiliary bimetal 15 has characteristics such that when it is properly thermally mounted, it will deflect in direction B when less than of the maximum rated value current passes thro-ugh the heater 13 and it will bend more slowly than the primary bimetal 11 in response to overload currents pass-ing through the heater. As with the primary bimetal, the auxiliary bimetal can be directly connected in series with the circuit being protected. The wiring of the circuit would lead into the mounted end of the auxiliary bimetal and out of the unmounted end thereof.

In the trip unit shown in FIGURE 1, the tripper bar 20 Iis actuated by lever 21. Lever 21 is attached to the tripper -bar 20. When the lever 21 is moved in the direction A by deflection of the primary .bimetal 11, the tripper bar 20 is rotated, which moves latch support plate 25, thereby tripping the circuit breaker in a manner more fully hereinbelow described.

Lever 21 has an abutment member 22 which extends therethrough. Abutment member 22 has side 26, designed to contact auxiliary bimetal 1S and side 27, designed to contact primary bimetal 11. Sides 26 and 27 of abutment member 22 are supported in position, respectively, by sleeves 28 and 29 atlixed to lever 21. Sides 26 and 27 may each have a threaded surface which mates with threaded apertures, respectively, in sleeves 28 and 29. Rotation of side 26 intsleeve 28 or of side 27 in sleeve 29, respectively, varies the proximity of each of these sides to auxiliary bimetal 15 or primary bimetal 11.

When primary bimetal 11 deects in direction A, it presses against end 23 of abutment member 22. When auxiliary bimetal 15 deects in direction B, it presses against end 24 of abutment member 22. The pressure exerted by auxiliary bimetal 15 is transferred through abutment member 22 to end 23 thereof.

When either of sides 26 or 27 of abutment member 22 is rotated in its threaded aperture in respective sleeve 28 or 29, either end 24 or end 23 of abutment member 22 is moved closer to or further from its related bimetal. In this manner, the percentage of deilection of each bimetal prior to its becoming operatively engaged with the abutment member 22 can be varied, thus Calibrating the trip unit for different `current ratings.

Auxiliary bimetal 15, in the present embodiment, is so comprised that it will begin its deflection at current levels below the maximum rating of the trip unit. When current at the maximum rated value flows for a period of time, for instance 21/2 hours, through the heater 13, this causes the primary bimetal 11 to move suciently in direction A to move abutment member 22, thereby rotating lever 21 which trips the breaker. Auxiliary bimetal 15, which begins its movement in direction B at current ratings below that of the maximum rated current value, will already have exerted force against the end 24 of abutment member 22 and will, therefore, be -preventing lever 21 from moving tripper bar 20. The slight pressure exerted by primary bimetal 11 when the breaker is operating at its maximum rated current value is insutlicient to overcome the force exerted by auxiliary bimetal 15. Hence, the abutment member 22 will not be moved in direction A and this will ensure that the circuit breaker will not be tripped.

At current ratings above the maximum rated current value of the trip unit, the faster moving primary Ibimetal 11 deflects and presses against end 23 of abutment member Z2, thereby disengaging actuating means 21 from tripper bar 20, which permits tripper bar 20 to operate the circuit breaker. Since the auxiliary bimetal 15 reacts more slowly than the primary bimetal 11 to overload currents, the auxiliary bimetal will not have deected sutiiciently before tripping -ot the unit to restrain movement of abutment member 22, and thus it will not restrain the movement in direction A of the actuating means 21.

The auxiliary bimetal 15 could be mounted so that it, like primary bimetal 11, detlects in direction A. The auxiliary bimetal 15 could then be positioned so that as it deflects it pus-hes against the primary bimetal 11 or it can `be mechanically linked with the primary bimetal 11 such that it pulls the primary bimetal 11 in direction A.

As another alternative, one end of the primary bimetal 11 might be amxed to the unmounted end of the auxiliary bimetal 15 so that the base of the primary bimetal 11 would be positioned at the point of maximum deflection of the unmounted end of the auxiliary bimettal 15. This type of mounting can be employed both in situations where the bimetals are mounted to deect in opposite directions or in situations where they are mounted to deflect in the same direction.

No matter how the bimetals are mounted, if the primary an-d auxiliary bimetals are both positioned to deflect in direction A, and if the auxiliary bimetal 15 is still designed to deflect at lower currents than the primary bimetal 11, then at low currents the auxiliary bimetal will cause t-he primary bimetal 11 to deflect more rapidly than the latter normally would without having any noticeable effect on the faster moving primary bimetal at higher overload currents. It should be understood that the primary and auxiliary bimetals can have such deflection characteristics and be so positioned that the auxiliary bimetal will have no effect on the primary bimetal at currents below overload while being so designed and positioned to affect, by either speeding up or slowing down, the dellection characteristic of the primary bimetal at certain overload currents.

The novel trip unit of FIGURE 1 having both a primary -bimetal and an auxiliary bimetal mounted on the heater is shown embodied in the circuit breaker 30 of FIGURE 2. No electromagnetic instantaneous trip mechan-ism as exists in most circuit breaker trip units is shown in FIGURE 2. For an example of a circuit breaker trip unit having both an instantaneous electromagnetic tripping mechanism and a tripping mechanism similar to the instant trip unit, refer to FIGURE 3 of U.S. Patent No. 3,213,232. The input current from the circuit being protected is fed to terminal 31. The current passes through heater extension 32, heater 13 and then heater extension 33. It then passes through conductor 34, conductive braid 35 and into movable contact arm 36. From there is passes through movable contact 37 to stationary contact 3S, along conductor 39 and into output terminal 40, which leads back into the circuit being protected.

Tripper bar has latch support plate 25 attached thereto. Latch support plate supports latch extension 41.

When an overload current passes through heater 13 and primary bimetal 11 deiiects in direction A striking abutment member 22 and rotating tripper bar 20, latch support plate 25 is rotated out from under latch extension 41 thereby permitting latch 43 to fall which, through intervening mechanical linkages, causes movable contact arm 36 to pull movable contact 37 away from stationary contact 38 opening the circuit being protected.

The movable Contact arm 36 is pivotally mounted at 45 between the arms of its individual U-shaped holder 46. Each holder 426 is provided with a portion 47 which defines a rectangular opening having an insulating tie rod 48 disposed therein. The present circuit breaker trip unit can be employed in multiphase, as well as single-phase, circuit breakers. Tie rod 48 extends through the housing compartments of each phase of the circuit being protected and interconnects the contact arm holders 46 of all phases to bring about the simultaneous movement of all three movable contact arms 36 responsive to tripping of one phase in the well-known manner. While the trip unit of the instant invention has been shown embodied in one type of circuit breaker, its usefulness is obviously not limited to this type of circuit breaker. It is adaptable for all other circuit breakers having a trip unit employing a thermal element to move the tripper mechanism.

Refer to FIGURES 3-5 which plot percent deflection of the bimetals as la function of time for different current ratings passing through the present embodiment of the trip unit.

In FIGURE 3, current at a value equal to of the rated value of the trip unit passes through the heater. Note that at 1% hours the auxiliary bimetal has heated suiciently tol begin to deect. The primary bimetal, however, does not begin to deflect until 2 hours have passed. Because the current passing through the heater is at 100% of the rated value of the trip unit, the primary bimetal does not deflect a great amount. The auxiliary bimetal, on the other hand, is so comprised that it would deflect to a great extent when current at 100% of the rated value of the trip unit passes through the heater. The resultant deflection prevents the primary bimetal from tripping the breaker.

In FIGURE 4, current at of the rated value of the trip unit is passing through the heater. The primary bimetal is so comprised as to react to this value of current after about 1/2 hour. Because the current passing through the heater has increased beyond 100%, the percent of deflection of the primary bimetal is also increased. The auxiliary bimetal also responds more rapidly at 125% of the rated value than it did at 100% of the rated value. It, therefore, begins detiecting after about 2%; of an hour. The auxiliary bimetal, however, is so comprised that its speed of movement is slower than that of the primary bimetal. The force exerted by the primary bimetal, due to its large deflection, is too great to be overcome by the auxiliary bimetal, and before the auxiliary bimetal is able to deflect sufficiently to counteract the force of the primary bimetal, the primary bimetal has tripped the circuit breaker. This tripping should occur after approximately one hour of operation at 125 of the rated value of the trip unit.

Refer to FIGURE 5 in which the current passing through the heater is at 300% of the maximum rated value of the trip unit. The primary bimetal reacts sharply to this large overload current by detlecting rapidly. The more slow moving auxiliary bimetal has not even begun to react when the primary bimetal deflects sufficiently to trip the circuit breaker. It takes approximately 15 seconds for the primary bimetal to deflect at 300% of the maximum rated value of the trip unit to trip the circuit breaker.

Although there has been described a preferred embodiment of this novel invention, many Variations and modications will now be apparent to those skilled in the art. Therefore, this invention is to be limited not by the specic disclosure herein, but only by the appending claims.

The embodiments of the invention in which an exclusive privilege or property is claimed are defined .as follows:

1. A circuit lbreaker trip unit for operation in conjunction with a circuit breaker; said circuit breaker including a pair of cooperating contacts and operating means for moving said contacts between an engaged and disengaged position; a heater, comprising a conductor in series with said cooperating contacts and connectable in series with the circuit being protected;

said trip unit including an actuating means connectable tothe operating means of said circuit breaker to effect movement of the circuit breaker contacts to their disengaged position;

said trip unit including thermally responsive trip means consisting of a primary and an auxiliary thermal element; each of said thermal elements being formed of a plurality of face bonded strips of material having diierent coefficients of thermal extension, whereby each of said thermal elements detlects due to increased temperature therein;

said primary and said auxiliary thermal elements each being mounted in direct engagement with said heater, enabling the heat in said heater to 'be transmitted directly by conduction to both of said thermal elements, whereby each ot said thermal elements is operatively responsive to the load current flow through said trip unit;

the coeicients of thermal expansion of the strips forming said primary ('bimetallic strip) thermal element being chosen to cause said primary thermal element to begin to operatively detiect at a Iiirst level .of said load current and to deflect to a first predetermined extent for each increase in temperature of said heater; the coefficients of thermal expansion of the strips forming said auxiliary thermal element being chosen to cause said auxiliary thermal element to begin to operatively deflect at a second level of said load current and to deflect to a second predetermined extent for each increase in temperature of said heater;

said thermal elements engaging said actuating means in a predetermined manner in accordance with their respective deection characteristics to provide a thermal trip unit characteristic operatively dependent on deection of both of said thermal elements.

2. The circuit breaker trip unit of claim 1 in which said primary and said auxiliary thermal elements deiiect in opposite directions, means linking said thermal elements such that the auxiliary thermal element alters the deiiection characteristics of the primary thermal element.

3. The circuit breaker trip unit of claim 2 in which said auxiliary thermal element and saidheater have a block of low thermal conductivity interposed therebetween for reducing the amount of heat passing from said heater to said auxiliary thermal element.

4. The circuit breaker trip unit of claim 2 in which said primary and said auxiliary thermal elements are positioned to deilect toward each other.

5. The circuit breaker trip unit of claim 4 in which said actuating means comprises a latch connected with a contact separating means for causing tripping apart of said cooperating contacts; said latch being normally biased to cause the said separating means to separate said cooperating contacts; a latch support supporting said latch against its normal bias, whereby said Separating means is prevented from operating; said latch support being secured to a rotatable tripper bar which is operatively engageable with said primary thermal element, whereby deliection of said primary thermal element rotates said tripper bar, which removes said latch support from beneath said latch, which permits said latch to assume its normally biased position thereby permitting said separating means to separate the said cooperating contacts.

6. The circuit breaker trip unit of claim f1, wherein said primary thermal element `begins deecting at a rst operative current value and said auxiliary thermal element begins deecting at a second operative current Value;

said second current value lesser than said first current value;

said primary thermal element characterized as deflecting more rapidly than said auxiliary thermal element when subjected to a current level above said lirst current value.

References Cited UNITED STATES PATENTS 2,318,279 5 1943 Aschwanden 200-116 2,683,199 7/1954 Spires 200--113 3,155,795 ll/l964 Jencks et al ZOO-116 BERNARD A. GILHEANY, Primary Examiner. H. B. GILSON, Assistant Examiner. 

